1. Field of the Invention
The present invention relates to an RFID system comprising a plurality of interrogators, and specifically relates to a technology to reduce influence caused by interference between the plurality of interrogators.
2. Description of the Related Art
A system such that ID information written in a tiny IC chip is read without touching the IC chip has been put to practical use in recent years. Since such a type of system usually utilizes radio wave in the radio-frequency band to transmit/receive signals, the system is referred to as RFID (Radio Frequency Identification) system.
An RFID system comprises an interrogator and a transponder. The interrogator accesses IC chips serving as a transponder and reads information (e.g. ID information) stored in the IC chips. The interrogator often writes information in the IC chips. For that reason, in the following description, the interrogator is referred to as a “reader/writer”. The transponder is an IC chip in which unique ID information is written, and is often referred to as various names such as an IC tag, a radio IC tag, an RFID tag, an electronic tag, and a radio tag. In the following description, however, the transponder is referred to as an “RF tag”.
In an RFID system, the reader/writer sends a radio signal (radio wave) for propagating interrogation information to an RF tag. The RF tag returns the information requested in the interrogation information to the reader/writer. Alternatively, the RF tag writes the information sent with the interrogation information in a memory. Note that the RF tag comprises a function for generating electrical power from a radio signal transmitted from the reader/writer. However it is also known that some RF tags incorporate batteries.
As described above, a radio signal is transmitted/received between the reader/writer and the RF tag in the RFID system. Particularly in the embodiment where electrical power is generated in an RF tag by using the radio signal transmitted by the reader-writer, the power of the radio signal transmitted from the reader/writer needs to be considerably large. For that reason, in the RFID system comprising a plurality of reader/writers provided proximity of each other, interference is caused between the reader/writers, and data reading function may be deteriorated. Here, “interference between reader/writers” is such that a signal from an RF tag in an interrogation area of one reader/writer receives influence of a signal transmitted from other reader/writer under an environment that a plurality of reader/writers use the same frequency.
As a technology for solving the problem, Patent Document 1, for example, describes a method of time-sharing control so that communication timing of a plurality of reader/writers does not overlap each other. However, although this method can prevent the interference between reader/writers from occurring, a plurality of reader/writers cannot perform communication simultaneously, accordingly the throughput of the whole system is deteriorated. In addition, Patent Document 1 assumes a system with only one channel between the reader/writer and the RF tag.
Patent Document 2 describes a configuration where a reader/writer detects interference wave (exogenous noise) in advance, and communication is controlled based on the result. However, although details of detection method of the interference wave are described, the document fails to describe details of how the influence of the interference wave can be removed when it is detected.
In addition, it is also possible to employ a method for preventing interference between reader/writers by introducing a frequency hopping. However, since the frequency hopping requires a wide bandwidth, it is sometimes difficult to adopt in Japan and Europe. In Europe, for example, the band assigned for RFID is 0.25 MHz (869.4-869.65 MHz of UHF band). A configuration using a directional antenna and a configuration in which radio wave shield is prepared are also possible to employ; however, any of above is not effective for reducing the cost.    Patent Document 1: Japanese Patent Application Publication No. 2003-283367 (Paragraphs 0002-0010, FIG. 8)    Patent Document 2: Japanese Patent Application Publication No. 2004-48288 (Paragraphs 0013, Abstract)